Many buildings and campuses are cooled by chilled water plants. In general, these chilled water plants use a chiller to produce chilled water, which is pumped to air handlers to cool building air. Chillers, air handlers, and other components of a chilled water plant are designed to operate at a specific chilled water entering and leaving temperature, or the difference, Delta T. At design Delta T, these components are at their most efficient and can produce cooling output at their rated capacity. Low Delta T, which occurs when the entering and leaving temperature become closer than the design Delta T, reduces efficiency and cooling capacity of the chilled water plant and causes the chilled water plant to use more energy than required for a given demand.
Many mitigation strategies have been developed to address Low Delta T, such as through the use of sophisticated sequencing programs and equipment ON/OFF selection algorithms. For example, chilled water plants using an air cooled chiller may cycle a condenser fan on and off or provide variable condenser fan speeds. As another example, variable chilled water flow is used. In most instances, the chilled water plant operator simply pumps more water to system air handlers to increase their output, but this has the compounding effect of further reducing the already low Delta T. Also, increased pumping results in higher than necessary pumping energy usage. These approaches may not completely resolve the low Delta T.
Air cooled chillers compound the inefficiency. Air cooled chillers are inefficient as compared to water cooled chillers. The condenser section in an air cooled chiller stores a low volume of liquid refrigerant, making low ambient temperature operation difficult because the refrigerant tends to build up in the condenser (i.e., stacking). This build-up causes system pressure to drop, and the chiller to shut down. The condenser fan section is generally set to maintain a constant condensing temperature. This may be ideal for a small range of ambient temperatures, but is usually too high or low at most times of the year. Nuisance trips may be caused by maintenance of the temperature.